i have an isuzu single cab pick up,i have my sound system in the other vahicle,and the box its like 7.5cuft ported.
the main problem now is that the bed of my truck its just 16-17inches in height..and my S15L7 just fit edge to edge,from the cover of the bed and the floor..

i dont like put it inverted,i just want it to face backwards..
the width and length of the box its not a problem..

what's the best enclosure for an s15L7 to go SPL?tuned @ what hz?

do i have to consider the volume of my truck's bed too?because its sealed..i mean,i have a flat cover made of marine wood..so its gonna be like a box in the box..i really dont know what to do..

you want the max spl from your 2x15"s with limited space,right?
well i'm not that good at this,so correct me if i'm wrong but i think max spl is attained if your box is tuned to the interior (effective) volume of your V..

sealed boxes are great for minimum space while a ported box have higher spl but are bigger & consumes a lot of space..
space is the price for paying high spl..that wil be your choice..

if you say SPL as:
you need the max output for db drag racing, you will need to tune your box to around 40-50Hz and sized at about 4.5-6cu ft depending on the sub T/S params. this would hit hard and sound 'matigas' and would typically sound like crap.

if you want very loud earth shaking lows that go really deep (malambot tumunog):
you need roughly the same size as above but tuned to around 30-40Hz for deeper bass extension. if you tune it this way, keep in mind you won't do well in a db drag event but great for everyday listening and 'street bass".